1. Field of the Invention
The present invention relates to a plant control system.
2. Description of the Prior Art
The applicant of the present application has proposed an air-fuel ratio control system having an exhaust gas sensor for detecting the concentration of a certain component of an exhaust gas that has passed through a catalytic converter such as a three-way catalytic converter disposed in the exhaust passage of an internal combustion engine, such as an O.sub.2 sensor for detecting the concentration of oxygen in the exhaust gas, the exhaust gas sensor being disposed downstream of the catalytic converter. The system controls the air-fuel ratio of the internal combustion engine, more accurately, the air-fuel ratio of an air-fuel mixture to be combusted by the internal combustion engine, in order to converge an output of the O.sub.2 sensor, i.e., the detected value of the oxygen concentration, to a predetermined target value for enabling the catalytic converter to have a desired purifying ability irrespective of the aging of the catalytic converter. See, for example, U.S. patent application Ser. No. 09/311353, U.S. patent application Ser. No. 09/153300, and Japanese patent application No. 11-93740 (U.S. patent application Ser. No. 09/153156).
According to the disclosed technology, the behavior of the exhaust system ranging from a position up-stream of the catalytic converter to the O.sub.2 sensor down-stream thereof is modeled. A manipulated variable for manipulating the air-fuel ratio of the internal combustion engine is successively generated in given control cycles in order to converge the output of the O.sub.2 sensor to its target value based on a feedback control process (specifically, a sliding mode control process) constructed based on the model. The air-fuel ratio of the internal combustion engine is manipulated by controlling operation of the internal combustion engine based on the manipulated variable, specifically, by adjusting the amount of fuel supplied to the internal combustion engine.
More specifically, the manipulated variable generated according to the feedback control process is a target value for the difference (hereinafter referred to as a "differential air-fuel ratio") between the actual air-fuel ratio of the internal combustion engine and a predetermined reference value (constant value) for the air-fuel ratio. According to the disclosed technology, an exhaust gas sensor (hereinafter referred to as an "air-fuel ratio sensor) for detecting the air-fuel ratio of the air-fuel mixture that has been burned by the internal combustion engine is disposed upstream of the catalytic converter. The amount of fuel supplied to the internal combustion engine is regulated according to a feedback control process so as to converge the output of the air-fuel ratio sensor, i.e., the detected value of the air-fuel ratio, to a target air-fuel ratio defined by the manipulated variable (a target value for the differential air-fuel ratio) and the reference value for thereby controlling the air-fuel ratio of the internal combustion engine at the target air-fuel ratio.
Such air-fuel ratio control for the internal combustion engine is capable of converging the output of the O.sub.2 sensor disposed downstream of the catalytic converter to its target value for thereby enabling the catalytic converter to have a desired purifying ability.
In the above proposed air-fuel ratio control system, the feedback control process using the output of the air-fuel ratio sensor disposed upstream of the catalytic converter is carried out for controlling the air-fuel ratio of the internal combustion engine at the target air-fuel ratio. However, it is also possible to control the air-fuel ratio of the internal combustion engine at the target air-fuel ratio according to a feed-forward control process by determining the amount of fuel supplied to the internal combustion engine from the target air-fuel ratio using a map or the like.
In the above air-fuel ratio control system, the O.sub.2 sensor is used as the exhaust gas sensor disposed down-stream of the catalytic converter. However, the exhaust gas sensor may comprise an NOx sensor, a CO sensor, an HC sensor, or another exhaust gas sensor. It is possible to enable the catalytic converter to have a desired purifying ability by controlling the air-fuel ratio of the internal combustion engine so as to converge the output of such an exhaust gas sensor to a suitable target value.
In the above conventional air-fuel ratio control system, the exhaust system, including the catalytic converter, which ranges from a position upstream of the catalytic converter to the O.sub.2 sensor downstream of the catalytic converter may be considered to be a plant for generating the output of the O.sub.2 sensor from the air-fuel ratio of the internal combustion engine (the air-fuel ratio as detected by the air-fuel ratio sensor). The internal combustion engine may be considered to be an actuator for generating an exhaust gas having an air-fuel ratio to be supplied to the plant. Thus, the air-fuel ratio control system may be expressed as a system for generating a manipulated variable to control the input (air-fuel ratio) to the plant (=an output from the actuator) to converge the output of the O.sub.2 sensor as the output of the plant to a given target value, and controlling operation of the internal combustion engine as the actuator based on the manipulated variable.
In modeling the exhaust system including the catalytic converter, the exhaust system is regarded as a system for generating the difference between the output of the O.sub.2 sensor and its target value with a response delay, etc. from the differential air-fuel ratio which is the difference between the air-fuel ratio of the air-fuel mixture combusted by the internal combustion engine and a predetermined fixed reference value with respect to the air-fuel ratio.
The feedback control process for generating the manipulated variable as the target value for the differential air-fuel ratio is constructed on the basis of the model of the exhaust system.
With the input (differential air-fuel ratio) to the exhaust system that is modeled and the output thereof (the difference between the output of the O.sub.2 sensor and its target value) being expressed as differences, the algorithm of the feedback control process for generating the manipulated variable can be simplified.
According to the above technique, a parameter for defining the behavior of the model of the exhaust system is successively identified using the output data of the air-fuel ratio sensor and the O.sub.2 sensor. In order to generate the manipulated variable, the feedback control process uses the identified parameter of the model of the exhaust system.
It has been found as a result of a further study by the inventors of the present application that when the output of the air-fuel ratio sensor suffers a steady offset from a normal output due to deterioration of the air-fuel ration sensor or when the actual air-fuel ratio is subject to a steady error with respect to the target air-fuel ratio due to an aging-induced characteristic change of the internal combustion engine, the quick response of a control process for converging the output of the O.sub.2 sensor based on the model of the exhaust system to the target value is lowered, and the output of the O.sub.2 sensor suffers a steady error with respect to the target value.
In another application, a manipulated variable for controlling an input to an arbitrary plant to converge a detected value of an output of the plant to a predetermined target value is generated according to a feedback control process based on a model of the plant which is constructed in the same manner as with the above technique. The above drawbacks are also caused in such an application when operation of the actuator for generating the input to the plant is controlled based on the manipulated variable.